Maybe I'm an unusual case, but I am a developer that does quite well at testing. Now to be fair, I am not the best developer out there, but I can develop my way out of a wet paper bag. :)
As a developer, when I was testing a bug fix I had just implemented, I would typically find 2 to 3 more bugs. That happened pretty much ever bug fix. My managers saw this and decided I would be better off in the QA department and I felt the same way.
Maybe the statement: "Great developers make bad testers" is better. I suspect that even great developers could make great testers too, they just don't feel it's worth their time to do both. That may be a true statement, but in the age of TDD, developers (good or bad) need to become great testers too.

Michael Hunter's blog byline is unapologetically over-the-top: making developers cry since 1995. That's probably why he's such an awesome tester. Well, that, and the braids. Never before in the history of testing professionals have the top and bottom halves of a man's head been so mismatched.*...